Bypass configured gas turbine engines such as those used on jet aircraft generally comprise an inlet fan for providing air to an engine core as well as to a bypass path around the core. Indeed, in some modern engine concepts, the bypass ratio can be as high as 17:1 and as much as 80% of the total thrust developed by the engine may be a result of the bypass airflow. These types of systems are referred to herein as bypass configured gas turbines. The engine core consists of an air inlet, a single or multi-stage compressor chamber, a combustion chamber aft (downstream) of the compressor chamber, a turbine and an exhaust nozzle. Air entering the inlet flows axially through the compressor chamber and into the combustion chamber where it provides oxygen for fuel ignition. At the same time, a much larger volume of air enters and passes through the annular bypass path around the core.
In a conventional (non-geared) bypass configured gas turbine engine, for a given fan size intended to provide a desired airflow, the tip speed required for the inlet fan fixes the fan rotational speed and thus fixes the low pressure shaft rotational speed. As the bypass ratio increases, i.e., as the ratio of the fan diameter and the core inlet diameter increases, the establishment of a desired fan tip speed can result in an inadequate mechanical speed for components attached to the fan rotor resulting in increased number of compression and turbine stages to deliver desired component characteristics.
However, in a geared bypass configured gas turbine engine (geared turbofan or GTF) engine, a reduction architecture is incorporated between the fan and the low pressure shaft. This allows the low pressure shaft to turn at a higher rotational speed without over speeding the inlet fan. As a result, a GTF engine does not require the use of additional turbine/compressor stages as in a conventional bypass configured gas turbine engine.
However, with the advantages provided by the gearing in a GTF engine, there are also additional challenges that must be overcome. For example, an incremental amount of heat energy is created via friction in the gear reduction architecture, and failure to effectively dissipate this heat energy can shorten the lifetime of gear reduction architecture components. While the bypass airflow may provide a tempting source of cooling air, this flow is very vulnerable to interference, especially as fan pressure ratio is decreased, and any obstructions may adversely affect the efficiency of the engine.
Thus there is a need for a GTF engine gear reduction architecture cooling system and technique that allow the use of bypass air flow for cooling while at the same time avoiding a substantial adverse impact on engine efficiency.